The present invention relates to optical disk apparatus for allowing an adequate servo control with respect to an optical disk for recording, reproducing and erasing data.
Optical disks are generally utilized as recording media which allow recording large quantities information and repeatedly reproducing and erasing the information. The optical disk apparatus illuminates, through an objective lens, a surface of an optical disk with light emitted from a semiconductor laser or the like so as to record an information signal on a spiral track formed on the optical disk surface or reproduce the information signal recorded thereon. Generally, the optical disk surface is impossible to be constructed to be perfectly flat, and the distance between the objective lens and the optical disk surface varies in accordance with the rotation of the optical disk due to the accuracy of the attachment mechanism and the like. In addition, since the depth of focus of the light spot is slight as compared with the variation of the distance, difficulty is encountered to accurately scan the signal-recorded surface of the optical disk with the light spot to thereby perform the normal information recording and reproducing operations. Accordingly, the position difference between the disk signal-recorded surface and the focus of the light spot is generally detected as the focus error signal so as to effect the focus servo control whereby the distance between the objective lens and the signal recorded-surface is kept constant. Furthermore, it is impossible that the center of the spiral track formed on the disk surface is arranged to be perfectly coincident with the center of the rotation of the optical disk, and therefore difficulty is encountered to accurately scan the center line of the track with the light spot due to the eccentricity to perform the normal recording and reproducing operations. Accordingly, the position difference between the center line of the track and the center of the light spot is generally detected as the tracking error signal so as to perform the tracking servo control whereby the scanning is allowed such that the center of the light spot is always coincident with the center line of the track. Moreover, for the recording and reproduction of data, the light spot is required to be accessible to the target position of the track. Thus, an access mechanism is required to move the optical pickup device at a high speed in the radial directions of the optical disk. Recently, the access control is actively studied.
The optical disk apparatus is required to have a frequency response characteristic up to several kHz for performing the servo control and further to be equipped with an actuator which is movable at least in a range between the outermost circumference and inner most circumference of the optical disk for the access control. Generally, a two-stage servo system is taken where a coil actuator or the like is used for the delicate movement and a linear motor or the like is used for the rough movement. However, as will be described hereinafter, the arrangement of such a conventional two-stage servo system can provide various problems such as the requirement of an actuator with a large stroke length and the difficulty of an adequate tracking control.